The present invention relates to a differential gearing for vehicle, and in particular, to a differential gearing for vehicle of a planetary gear type with a torque responsive differential action limiting function or torque distribution function.
A differential gearing for vehicle of a planetary gear type comprising an internal gear having internal teeth formed around an internal peripheral surface thereof, a sun gear disposed on a circle located inside of and concentric with the internal gear, a plurality of planet gears disposed between the internal gear and the sun gear in meshing engagement with the both gears, and a planetary carrier for carrying the planet gears for rotation about the axis of rotation of the internal gear and the sun gear while permitting the planet gears to rotate about their own axes with an arrangement that a shaft which transmits a drive from an engine is connected to the planetary carrier while the internal gear and the sun gear are connected to front and rear wheels of a four wheel driven vehicle, for example, is known in the art.
FIG. 11 shows an exemplary differential gearing for vehicle of a planetary gear type mentioned above and a conventional differential gearing will be described briefly with reference to FIG. 11. A planetary carrier 102 and a case 104 are secured together to define a housing 108 in which an internal gear 110 is received, and a sun gear 112 is disposed on a circle located inside of and concentric with the internal gear 110. A plurality of planet gears 114, which are carried by the planetary carrier 102, are disposed between the internal gear 110 and the sun gear 112 in meshing engagement with the both gears 110 and 112. In the arrangement shown in FIG. 11, the internal gear 110, the sun gear 112 and the planet gears 114 have helical gear teeth which are in meshing engagement with one another.
While not shown, the planetary carrier 102 has a cylindrical portion which extends into the inside of the internal gear 110 and which is formed with a plurality of openings at a given interval in which the planet gears 114 are received, respectively, in a rotatable manner. When the planet gears 114 rotate about their own axes, the tooth tips thereof slide against the internal surface of the openings.
The internal gear 110 has a cylindrical portion 110b and a flange 110c which extends radially inward from one end (right end as viewed in FIG. 11) of the cylindrical portion 110b. Internal teeth 110a are formed around the inner peripheral surface of the cylindrical portion 110b while splines 110d (see FIG. 13) are formed around the inner peripheral surface of the flange 110c. Splines 110d in the flange 110c are engaged by splines 116a formed around the outer periphery of a coupling 116. The splines 110d and 116a which connect the internal gear 110 and the coupling 116 together to transmit a power have been straight splines which extends parallel to an axis L3 in a conventional arrangement.
Washers 120, 118 and 124 are interposed between one end face (right end face as viewed in FIG. 11) of the coupling 116 which is connected with the internal gear 110 through the straight splines and the internal surface of the case 104, between the other end face of the coupling 116 and the end face of the sun gear 112, and between the other end face of the sun gear 112 and the internal surface of the planetary carrier 102. Washers 128 and 126 are interposed between the external surface (right-hand surface as viewed in FIG. 11) of the flange 110c of the internal gear 110 and the internal surface of the case 104 and between the internal surface of the flange 110c and the tip end face of the planet gears 114.
In the differential gearing for vehicle constructed in the manner mentioned above, a drive from an engine is transmitted to the planetary carrier 102 while one of front and rear wheels of a four wheel driven vehicle is connected to splines 116b formed around the inner periphery of the coupling 116 which is spline-connected to the internal gear 110 and the other wheel is connected to splines 112b formed around the inner periphery of the sun gear 112.
In the differential gearing for vehicle, when an drive from an engine is transmitted, the planetary carrier 102 which carries the planet gears 114 is driven for rotation, and if the vehicle is running straightforward on a good road surface, the planet gears 114 carried by the planetary carrier 102 as well as the internal gear 110 and the sun gear 112 which mesh with the planet gears 114 rotate in an integral manner without a relative rotation therebetween. When cornering, the planet gears 114 carried by the planetary carrier 102 rotate about their own axes, thereby correcting for a differential rotation of the front and the rear drive wheel which are connected to the internal gear 110 and the sun gear 112, respectively.
When the coefficients of friction which the front and the rear wheel experience from the road surface are different to produce a difference in the grip of the front and the rear drive wheel, a torque distribution function or a function to limit the differential action is provided by a frictional force which results from a sliding motion of the tooth tip of the planet gears 114 which are held within the gear holding openings formed in the planetary carrier 102 to rotate against the internal peripheral surface of the openings, and by a frictional force utilizing thrusts resulting from the meshing engagement between the helical gear teeth of the internal gear 110 and the planet gears 114 and the sun gear 112.
For example, when an input from the engine is applied to the planetary carrier 102 while an output from the internal gear 110 is applied to the rear wheel and an output from the sun gear 112 is applied to the front wheel as mentioned previously, it follows that in the drive mode, a thrust (see an arrow A shown in FIG. 11) which results from the meshing engagement between the planet gears 114 and the internal gear 110 acts on the washer 128 disposed on the end of the internal gear 110 located toward the case 104 to produce a frictional force, and a thrust (see an arrow B in the same Figure) which results from the meshing engagement between the planet gears 114 and the sun gear 112 acts on the washer 124 disposed on the end of the sun gear 112 located toward the planetary carrier 102 to produce a frictional force. In this instance, the locations where the frictional forces are developed are limited to the washers 128 and 124, resulting in a low bias ratio (torque distribution ratio) obtained.
During the coasting mode (see FIG. 12), that is a condition of a run by inertia, a thrust (see an arrow C) which results from the meshing engagement between the planet gears 114 and the internal gear 110 acts on the washer 126 located between the internal gear 110 and the planet gears 114 to produce a frictional force, and a thrust (see an arrow D) which results from the meshing engagement between the planet gears 114 and the sun gear 112 acts on the washer 118 disposed between the sun gear 112 and the coupling 116 to produce a frictional force. In addition, a thrust from the sun gear 112 is applied to the intermediate washer 118 to produce a frictional force upon the washer 120 disposed between the coupling 116 and the internal surface of the case 104, thereby allowing a bias ratio to be obtained which is of a median level greater than the bias ratio obtained during the drive mode.
In the differential gearing for vehicle of the prior art which is constructed in the manner mentioned above, an approach is available which adjusts the bias ratio in accordance with the helix angle of the helical gear in order to obtain a bias ratio which is desired depending on the characteristic of the vehicle. However, a manufacturing difficulty is involved in achieving a desired helix angle, and it has been difficult to obtain an optimum bias ratio according to such adjustment approach alone. In particular, in order to improve a limit on the drive and the dynamic behavior of the vehicle, it is necessary to achieve a higher bias ratio, but a sufficiently high bias ratio cannot be obtained with a conventional approach.
At this end, a differential gearing with an arrangement to achieve a high bias ratio (a differential action limiting function) has been proposed (see Japanese Laid-Open Patent Application No. 9-144,844). According to the differential gearing disclosed in this cited Patent Application, a multiple-disk clutch acting as means which intensifies the force limiting the differential action is disposed between the internal gear and the planet gears (which are referred to as pinion gears in the cited Patent Application).